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          <h1 class="post-title" itemprop="name headline">Java NIO学习笔记（三）Buffer</h1>
        

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        <p>Java NIO缓冲区用于与NIO通道进行交互。这是写入数据的内存块，以便在稍后再次进行读取。 内存块用NIO缓冲对象包装，这样可以提供更简单的方法来处理内存块。</p>
<h3 id="Buffer的类型"><a href="#Buffer的类型" class="headerlink" title="Buffer的类型"></a>Buffer的类型</h3><p>对于每个原始类型，都有一个缓冲区类型，所有缓冲区都可以实现缓冲区接口。 大多数使用的缓冲区类型是ByteBuffer。<br>在Java NIO中使用的核心缓冲区如下：</p>
<ul>
<li>ByteBuffer</li>
<li>CharBuffer</li>
<li>DoubleBuffer</li>
<li>FloatBuffer</li>
<li>IntBuffer</li>
<li>LongBuffer</li>
<li>ShortBuffer</li>
</ul>
<p>这些Buffer覆盖了能通过IO发送的基本数据类型：byte, short, int, long, float, double 和 char。<br>Java NIO 还有个 MappedByteBuffer，用于表示内存映射文件。</p>
<img src="/blog/NIO_buffer/1.png">
<h3 id="Buffer的分配"><a href="#Buffer的分配" class="headerlink" title="Buffer的分配"></a>Buffer的分配</h3><p>要想获得一个Buffer对象首先要进行分配。 每一个Buffer类都有一个allocate方法。下面是一个分配48字节capacity的ByteBuffer的例子。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">ByteBuffer buf = ByteBuffer.allocate(<span class="number">48</span>);</span><br></pre></td></tr></table></figure>
<h3 id="Buffer的用法"><a href="#Buffer的用法" class="headerlink" title="Buffer的用法"></a>Buffer的用法</h3><p>使用Buffer读写数据一般遵循以下四个步骤：</p>
<ul>
<li>写入数据到Buffer</li>
<li>调用flip()方法切换为读模式</li>
<li>从Buffer中读取数据</li>
<li>调用clear()方法或者compact()方法</li>
</ul>
<p>当向buffer写入数据时，buffer会记录下写了多少数据。一旦要读取数据，需要通过flip()方法将Buffer从写模式切换到读模式。在读模式下，可以读取之前写入到buffer的所有数据。<br>下面是一个使用Buffer的例子：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br></pre></td><td class="code"><pre><span class="line">RandomAccessFile aFile = <span class="keyword">new</span> RandomAccessFile(<span class="string">"data/nio-data.txt"</span>, <span class="string">"rw"</span>);</span><br><span class="line">FileChannel inChannel = aFile.getChannel();</span><br><span class="line"> </span><br><span class="line"><span class="comment">//create buffer with capacity of 48 bytes</span></span><br><span class="line">ByteBuffer buf = ByteBuffer.allocate(<span class="number">48</span>);</span><br><span class="line"> </span><br><span class="line"><span class="keyword">int</span> bytesRead = inChannel.read(buf); <span class="comment">//read into buffer.</span></span><br><span class="line"><span class="keyword">while</span> (bytesRead != -<span class="number">1</span>) &#123;</span><br><span class="line"> </span><br><span class="line">  buf.flip();  <span class="comment">//make buffer ready for read</span></span><br><span class="line"> </span><br><span class="line">  <span class="keyword">while</span>(buf.hasRemaining())&#123;</span><br><span class="line">    System.out.print((<span class="keyword">char</span>) buf.get()); <span class="comment">// read 1 byte at a time</span></span><br><span class="line">  &#125;</span><br><span class="line"> </span><br><span class="line">  buf.clear(); <span class="comment">//make buffer ready for writing</span></span><br><span class="line">  bytesRead = inChannel.read(buf);</span><br><span class="line">&#125;</span><br><span class="line">aFile.close();</span><br></pre></td></tr></table></figure>
<p>缓冲区本质上是一块可以写入数据，然后可以从中读取数据的内存。这块内存被包装成NIO Buffer对象，并提供了一组方法，用来方便的访问该块内存。</p>
<h4 id="向Buffer中写数据"><a href="#向Buffer中写数据" class="headerlink" title="向Buffer中写数据"></a>向Buffer中写数据</h4><p>写数据到Buffer有两种方式：</p>
<ul>
<li>从Channel写到Buffer。</li>
<li>通过Buffer的put()方法写到Buffer里。</li>
</ul>
<p>BufferedReader从testout.txt文件中读取行的简单示例：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">int</span> bytesRead = channel.read(buf); <span class="comment">//read into buffer.</span></span><br></pre></td></tr></table></figure></p>
<p>通过put方法写Buffer的例子：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">buf.put(<span class="number">127</span>);</span><br></pre></td></tr></table></figure></p>
<p>put方法有很多版本，允许以不同的方式把数据写入到Buffer中。例如， 写到一个指定的位置，或者把一个字节数组写入到Buffer。</p>
<h4 id="从Buffer中读取数据"><a href="#从Buffer中读取数据" class="headerlink" title="从Buffer中读取数据"></a>从Buffer中读取数据</h4><p>从Buffer中读取数据有两种方式：</p>
<ul>
<li>从Buffer读取数据到Channel。</li>
<li>使用get()方法从Buffer中读取数据。</li>
</ul>
<p>从Buffer读取数据到Channel的例子：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//read from buffer into channel.</span></span><br><span class="line"><span class="keyword">int</span> bytesWritten = inChannel.write(buf);</span><br></pre></td></tr></table></figure>
<p>使用get()方法从Buffer中读取数据的例子</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">byte</span> aByte = buf.get();</span><br></pre></td></tr></table></figure>
<p>get方法有很多版本，允许以不同的方式从Buffer中读取数据。例如，从指定position读取，或者从Buffer中读取数据到字节数组。</p>
<h3 id="Buffer的属性"><a href="#Buffer的属性" class="headerlink" title="Buffer的属性"></a>Buffer的属性</h3><p>为了理解Buffer的工作原理，需要熟悉它的三个属性：</p>
<ul>
<li><strong>capacity</strong><br>作为一个内存块，Buffer有一个固定的大小值，也叫“capacity”.你只能往里写capacity个byte、long，char等类型。一旦Buffer满了，需要将其清空（通过读数据或者清除数据）才能继续写数据往里写数据。</li>
<li><strong>position</strong><br>当写数据到Buffer中时，position表示当前的位置。初始的position值为0.当一个byte、long等数据写到Buffer后， position会向前移动到下一个可插入数据的Buffer单元。position最大可为capacity – 1.<br>当读取数据时，也是从某个特定位置读。当将Buffer从写模式切换到读模式，position会被重置为0. 当从Buffer的position处读取数据时，position向前移动到下一个可读的位置。</li>
<li><strong>limit</strong><br>在写模式下，Buffer的limit表示你最多能往Buffer里写多少数据。 写模式下，limit等于Buffer的capacity。<br>当切换Buffer到读模式时， limit表示你最多能读到多少数据。因此，当切换Buffer到读模式时，limit会被设置成写模式下的position值。换句话说，你能读到之前写入的所有数据（limit被设置成已写数据的数量，这个值在写模式下就是position）</li>
</ul>
<p>如图：</p>
<img src="/blog/NIO_buffer/2.png">
<h3 id="Buffer的常用方法"><a href="#Buffer的常用方法" class="headerlink" title="Buffer的常用方法"></a>Buffer的常用方法</h3><h4 id="flip-方法"><a href="#flip-方法" class="headerlink" title="flip()方法"></a>flip()方法</h4><p>flip方法将Buffer从写模式切换到读模式。调用flip()方法会将position设回0，并将limit设置成之前position的值。<br>换句话说，position现在用于标记读的位置，limit表示之前写进了多少个byte、char等 —— 现在能读取多少个byte、char等。</p>
<h4 id="rewind-方法"><a href="#rewind-方法" class="headerlink" title="rewind()方法"></a>rewind()方法</h4><p>Buffer.rewind()将position设回0，所以你可以重读Buffer中的所有数据。limit保持不变，仍然表示能从Buffer中读取多少个元素。</p>
<h4 id="clear-与compact-方法"><a href="#clear-与compact-方法" class="headerlink" title="clear()与compact()方法"></a>clear()与compact()方法</h4><p>一旦读完Buffer中的数据，需要让Buffer准备好再次被写入。可以通过clear()或compact()方法来完成。<br>如果调用的是clear()方法，position将被设回0，limit被设置成 capacity的值。换句话说，Buffer 被清空了。Buffer中的数据并未清除，只是这些标记告诉我们可以从哪里开始往Buffer里写数据。</p>
<p> 如果Buffer中仍有未读的数据，且后续还需要这些数据，但是此时想要先先写些数据，那么使用compact()方法。<br>compact()方法将所有未读的数据拷贝到Buffer起始处。然后将position设到最后一个未读元素正后面。limit属性依然像clear()方法一样，设置成capacity。现在Buffer准备好写数据了，但是不会覆盖未读的数据。</p>
<h4 id="mark-与reset-方法"><a href="#mark-与reset-方法" class="headerlink" title="mark()与reset()方法"></a>mark()与reset()方法</h4><p>通过调用Buffer.mark()方法，可以标记Buffer中的一个特定position。之后可以通过调用Buffer.reset()方法恢复到这个position。例如：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">buffer.mark();</span><br><span class="line"></span><br><span class="line">buffer.reset();  <span class="comment">//set position back to mark.</span></span><br></pre></td></tr></table></figure>
<h4 id="equals-与compareTo-方法"><a href="#equals-与compareTo-方法" class="headerlink" title="equals()与compareTo()方法"></a>equals()与compareTo()方法</h4><p>可以使用equals()和compareTo()方法比较两个Buffer。</p>
<ul>
<li><p><strong>equals()方法</strong><br>当满足下列条件时，表示两个Buffer相等：</p>
<ul>
<li>有相同的类型（byte、char、int等）。</li>
<li>Buffer中剩余的byte、char等的个数相等。</li>
<li>Buffer中所有剩余的byte、char等都相同。</li>
</ul>
<p>equals只是比较Buffer的一部分，不是每一个在它里面的元素都比较。实际上，它只比较Buffer中的剩余元素。</p>
</li>
<li><strong>compareTo()方法</strong><br>compareTo()方法比较两个Buffer的剩余元素(byte、char等)， 如果满足下列条件，则认为一个Buffer“小于”另一个Buffer：<ul>
<li>第一个不相等的元素小于另一个Buffer中对应的元素 。</li>
<li>所有元素都相等，但第一个Buffer比另一个先耗尽(第一个Buffer的元素个数比另一个少)。</li>
</ul>
</li>
</ul>

      
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